It is well known in the art that polymers, such as polytetrafluoroethylene (PTFE), are used to form a prosthesis. A tubular graft may be formed by stretching and expanding PTFE into a structure referred to as expanded polytetrafluoroethylene (ePTFE). Tubes formed of ePTFE exhibit certain beneficial properties as compared with textile prostheses. The expanded PTFE tube has a unique structure defined by nodes interconnected by fibrils. The node and fibril structure defines pores that facilitate a desired degree of tissue ingrowth while remaining substantially fluid-tight. Tubes of ePTFE may be formed to be exceptionally thin and yet exhibit the requisite strength necessary to serve in the repair or replacement of a body lumen. The thinness of the ePTFE tube facilitates ease of implantation and deployment with minimal adverse impact on the body.
While exhibiting certain superior attributes, ePTFE material is not without certain disadvantages. One disadvantage is the porosity of the ePTFE structure which permits cellular ingrowth. The ingrowth is undesirable if one uses the ePTFE material as a temporary graft to bridge vessels and it is desired to have clear access to the ePTFE graft for replacement or removal.
U.S. Pat. No. 5,665,114 to Weadock et al. discloses an implantable prosthesis made of ePTFE wherein the pores are filled with an in situ cross-linkable biocompatible and biodegradable material. The bio-material may be applied to the ePTFE using force to fill the pores with a dispersion or solution of the biomaterial, which is subsequently insolubilized therein.
U.S. Pat. No. 5,152,782 to Kowligi et al. discloses a non-porous elastomeric coating on a PTFE graft. The elastomeric coating is made of polyurethanes or silicone rubber elastomers. The elastomeric coating is applied to the graft by radially expanding the PTFE graft, and dipping or spraying the graft with the elastomeric coating. The radial expansion is controlled to ensure that the polymer coating penetration is restricted to the outer layers of the PTFE tube.
Another disadvantage is the ePTFE material has a tendency to leak blood at suture holes and often propagate a tear line at the point of entry of the suture. The suture holes in ePTFE do not self-seal due to the inelasticity of ePTFE material. As a result, numerous methods of orienting the node and fibril structure have been developed to prevent tear propagation. These processes are often complicated and require special machinery and/or materials to achieve this end. Prior art suggests encapsilling the ePTFE material with a liquid elastomer layer, the elastomer fills in and seals that suture hole.
U.S. Pat. No. 5,192,310 to Herweck et al. discloses a self-sealing PTFE or ePTFE vascular graft having a primary and secondary lumen. The primary lumen is to accommodate blood flow. The secondary lumen shares the outer wall as a common wall with the primary lumen. The secondary lumen is filled with a non-biodegradable elastomer material, such as silicone rubber, polyurethane, polyethers or fluoropolymers.
U.S. Pat. No. 5,904,967 to Ezaki et al. discloses a puncture resistant bio-compatible medical material for use on as grafts or artificial blood vessels. The puncture resistant material is sandwiched between two porous layers of the graft. The porous layers may be made of polyester resin or polyethylene terephthalate/polybuthylene terephthalate. The puncture resistant layer may be a styrene and/or olefin elastomer or isoprene derivatives. The layers are bonded by an adhesive or by fusion with heat.
Another problem is that ePTFE material exhibits a relatively low degree of longitudinal compliance. Expanded PTFE is generally regarded as an inelastic material. It has little memory and stretching results in deformation. In those instances when a surgeon will misjudge the length of the graft that is required to reach between the selected artery and vein, the surgeon may find that the graft is too short to reach the targeted site once the graft has been tunneled under the skin. Expanded PTFE vascular grafts typically exhibit minimal longitudinal compliance, and hence the graft does not stretch significantly along its longitudinal axis. Accordingly, in such cases, the surgeon must then remove the tunneled graft from below the skin and repeat the tunneling procedure with a longer graft.
Elasticity of an ePTFE vascular graft is important when used for bypass implants such as an axillofemoral bypass graft, wherein the vascular graft extends between the femoral artery in the upper leg to the axillary artery in the shoulder, as well as a femoropopliteal bypass graft extending below the knee. Such bypass grafts often place restrictions upon the freedom of movement of the patient in order to avoid pulling the graft loose from its anchor points. For example, in the case of the axillofemoral bypass graft, sudden or extreme movements of the arm or shoulder must be entirely avoided. Similarly, in the case of the femoropopliteal bypass graft, bending the knee can place dangerous stress upon the graft. The above-described restricted movement is due largely to the inability of the ePTFE vascular graft to stretch along its longitudinal axis when its associated anchor points are pulled apart from one another. Such restrictive movement is especially important during the early period of healing following implantation when there is still little tissue incorporation into the graft and it can move within the subcutaneous tunnel.
It is desirable to incorporate elastomeric properties into the PTFE. This incorporation is difficult because PTFE is a hydrophobic material making it difficult to wet with the hydro-based elastomers, and the elastomers are hydrophilic making them naturally attracted to other elastomeric molecules. When elastomeric material is applied to PTFE the two materials repel each other and the elastomer flow away from the nodes to a less hydrophobic area, the pores. The pores between the fibrils, are too small for the elastomeric material to penetrate. Thus, the elastomer remains on the surface of the fibrils and coat the exterior of PTFE.
Prior art suggests surface coating the ePTFE material with elastomer by dipping, spraying, or adhesive bonding. One disadvantage is that the coating may flake or separate from the ePTFE material, as well as add to the thickness of the ePTFE material.
U.S. Pat. No. 4,304,010 to Mano discloses a tubular prosthesis which is made of PTFE with a porous elastomeric coating on the outer surface. The elastomeric coating, which may be cross-linked, is described as being fluorine rubber, silicone rubber, urethane rubber, acrylic rubber or natural rubber, and may be applied to the PTFE prosthesis by wrapping, dipping, spraying or use of negative pressure.
U.S. Pat. No. 5,026,591 to Henn et al. discloses a coating product which contains a substrate and scaffolding, such as PTFE or ePTFE, where the pores are filled with a thermoplastic or thermosetting resin. The substrate may be of a diverse selection; i.e., woven, non-woven, fabric, paper, or porous polymer. Application of the resin to the PTFE substrate uses rollers to provide a controlled even coating.
U.S. Pat. No. 5,653,747 to Dereume discloses a stent to which a graft is attached. The graft component is produced by extruding polymer in solution into fibers from a spinnerette onto a rotating mandrel. A stent may be placed over the fibers while on the mandrel and then an additional layer of fibers spun onto the stent. The layer of layers of fibers may be bonded to the stent and/or one another by heat or by adhesives. The porous coating may be made from a polyurethane or polycarbonate urethane which may be bonded by heat or by adhesion to the support.
U.S. Pat. No. 4,321,711 to Mano discloses a vascular prosthesis of PTFE with an anti-coagulant coating and bonded to its outer surface a porous elastomer coating containing a coagulant. The elastomer is used in its crosslinked state and is made of fluorine rubber, silicone rubber, urethane rubber, acrylic rubber or natural rubber. The elastomeric coating is bonded to the PTFE by dipping, spraying and/or applying negative pressure to inside wall PTFE to pass elastomer through the wall.
U.S. Pat. No. 4,955,899 to Della Conna et al. discloses a longitudinally compliant PTFE graft. The PTFE tube is longitudinally compressed and the outer wall of the PTFE is coated with a biocompatible material, such as polyurethanes or silicone rubber elastomers. The coating is applied by compressing the PTFE tube on a mandrel, and dipping or spraying the PTFE with the elastomer. The elastomer coating is restricted to the outer layers of the PTFE tube. The elastomer coated PTFE is dried while in the compressed state.
Other prior art suggests bonding a separate layer of elastomer to the ePTFE material to enhance the elasticity. One disadvantage is the added thickness of the PTFE. Another disadvantage, as stated above with the elastomer coatings, is the layers will separate over time and can flake off the PTFE. Examples of bonding layers of elastomer to PTFE is discussed below.
U.S. Pat. No. 4,816,339 to Tu et al. discloses a bio-compatible material made from layers of PTFE and hydrophobic PTFE fibers coated with an elastomer mixture. The bio-compatible material disclosed is a PTFE layer, elastomer/PTFE mixed layer, elastomer layer and hydrophilic monomer fibrous elastomer matrix layer. The elastomer layer is made from polyurethane. The elastomer is applied to the combined PTFE layer by heating and radially expanding the combined PTFE layers and dipping or spraying the combined PTFE layers with elastomer.
U.S. Pat. No. 5,628,782 to Myers et al. discloses a biocompatible base material such as PTFE or ePTFE with an outer deflectably secured outer covering. The preferably outer covering is non-elastic porous film or fibers, preferable PTFE. The outer covering is secured to the base by use of an adhesive.
U.S. Pat. No. 6,156,064 to Choumard discloses a braided self-expandable stent-graft-membrane. This three layer invention has an interior graft layer which is braided PET, PCV or PU fibers; a middle layer which is the stent; and an exterior membrane layer which is a silicone or polycarbonate methane. The membrane layer is applied to the exterior of the stent layer by dipping, by braiding, by spraying or by fusing; which includes use of adhesive, solvent bonding or thermal and/or pressure bonding.
It is desirable to provide an ePTFE material that achieves many of the above-stated benefits without the resultant disadvantages associated therewith and disadvantages of similar conventional products. It is also desirable to make this elastomerically recoverable PTFE material available to be manufactured in a variety of used such as an implantable prosthesis, patch material, graft, or stent.